1. Field
We describe an apparatus and a method of decoding a SECAM composite video baseband signal and, more particularly, to an apparatus and method of recovering a chrominance signal from a SECAM composite video baseband signal.
2. Related Art
A video signal may be compliant with various broadcast standards corresponding to various industry groups, including the National Television System Committee (NTSC), Phase Alternating Line (PAL), and Séquentiel Couleur Avec Mémoire (SECAM).
The SECAM standard is the composite video signal standard that was developed in France in 1953. SECAM is an analog color broadcast television system primarily used in France, Russia, and Eastern Europe.
The SECAM standard has 625 scan lines per frame in a 6-MHz bandwidth, 25 frames per second, and 50 fields per second in a 2:1 interlaced system. The definitions of luminance and chrominance are the same with the PAL standard, except for bandwidth.
While the NTSC and the PAL standards transmit the chrominance signal in a Quadrature Amplitude Modulation (QAM) scheme, the SECAM standard transmits the chrominance signal in a Frequency Modulation (FM) scheme. In addition, the SECAM standard separately transmits an R-Y color difference signal and a B-Y color difference signal through alternate scan lines. That is, in one scan line, the R-Y color difference signal is transmitted, while the B-Y color difference signal is ignored. In a next scan line, the B-Y color difference signal is transmitted, while the R-Y color difference signal is ignored. Accordingly, the vertical color resolution decreases by half. Compared with the NTSC and the PAL standards, the SECAM standard prevents chrominance signal interference, resulting in more stable color reproduction.
FIG. 1 is a frequency spectrum of a SECAM composite video baseband signal.
Referring to FIG. 1, a luminance signal (Y) occupies an entire video bandwidth. In contrast, an R-Y color difference signal (referred to as a DR chrominance signal) and a B-Y color difference signal (referred to as a DB chrominance signal) are FM modulated on their inherent sub-carrier frequencies.
The DR chrominance signal has a sub-carrier frequency (FOR) of about 4.40625 of MHz and the DB chrominance signal sub-carrier frequency (FOB) has about 4.25 of MHz.
In FIG. 1, the DR chrominance signal and the DB chrominance signal are alternately transmitted through the scan lines. For example, the DR chrominance signal is transmitted through the odd lines of a field and the DB chrominance signal is transmitted through the even lines of the same field.
As described above, like the NTSC and the PAL standards, the SECAM standard transmits the chrominance signal and the luminance signal over one channel.
The luminance signal alone exists in a relatively low frequency band, the luminance signal and the chrominance signal coexist in a medium frequency band, and again the luminance signal alone exists in a relatively high frequency band.
In the transmission scheme, no reference signal is used to discriminate between luminance and chrominance signals. The luminance signal and chrominance signals may mix to produce chrominance noise. More particularly, the luminance signals existing in a region in which the chrominance signal exists or in close proximity to the region may be mixed with the chrominance signals to produce chrominance noise.
The FM chrominance signal represents colors at various frequencies. To decode the FM chrominance signal into the original chrominance signal, a phase change must be detected in real time and a signal must be determined depending on the phase change.
Accordingly a need remains for an improved apparatus and method for decoding SECAM chrominance signals.